Understanding how cells receive chemical signals from their surrounding environment and transform them into appropriate responses is a fundamental problem in biology. A significant challenge in studying this signal processing is controlling the cellular input signals with sufficient precision such that mathematical frameworks, e.g. control theory, can be more readily applied to their modeling and analysis. To address this problem, we have developed and experimentally validated a microfluidic device that can deliver chemical treatments in discrete packets, or plugs, to cells with high spatiotemporal precision. Using monolithic valves, we can insert a chemical plug in the channel and flow it past the cells. Taylor dispersion morphs the initial well-defined plug into a Gaussian curve. Application of the device was demonstrated by exposing human epidermal keratinocytes, skin cells, to plugs of ethanol and examining cell viability versus exposure time. Results suggest that this device can be used for controlled cell stimulation.